Lunar dynamo Scientists may have solved one of the Moon's enduring mysteries: why there are magnetised rocks on the lunar surface, when the Moon has no global magnetic field.

Lunar rock samples brought back by the Apollo astronauts provided evidence of an ancient magnetic field on the Moon about 100 times weaker than the Earth's magnetic field.

On Earth, heat from the solid inner core drives complex fluid dynamics through convection in the planet's molten outer core, causing an electrical charge which generates a global magnetic field.

Scientists originally thought a similar process may have generated an early lunar magnetic field. However the small size of the Moon's core and the required surface magnetic field strengths were problematic for this theory.

Now, two new studies published in the journal Nature have come up with possible explanations for a magnetic lunar field.

Lunar dynamo

A paper by scientists including Christina Dwyer from the University of California Santa Cruz, says an ancient geodynamo could have existed in the Moon's distant past.

Dwyer and colleagues say the Moon's magnetic field was generated because its liquid core once rotated differently to its overlaying solid mantle.

This differential motion was caused by gravitational interaction when the Moon was much closer to Earth, causing a wobble in the lunar axis as it spun.

Dwyer and colleagues say this lunar dynamo could have operated in this way for at least a billion years, shutting down only when the Moon receded far enough from the Earth.

"This is a very different way of powering a dynamo that involves physical stirring, like stirring a bowl with a giant spoon," says Dwyer.

Astronomer, Dr Brad Carter from the University of Southern Queensland says the paper correctly predicts the level of magnetic field intensity seen in the Moon rock samples.

"The theory is sufficiently flexible to be able to tweak the history of the Moon's magnetic field and tell us something about the Moon's orbital history as well, "says Carter.

Impact theory

The second paper by scientists including Dr Michael Le Bars from the IRPHE, CNRS and Aix-Marseille Universite in France, propose that large repeated impacts events early in the Moon's history may have changed its rotation rate, causing tidal distortion at the core-mantle boundary.

Le Bars and colleagues say this may cause differential movements of the core, thereby powering a transient lunar dynamo.

They say six large impact basins with central magnetic anomalies offer clues to the origin of lunar magnetism. Each of these impact events would also have significantly affected the rotational state of the Moon, causing the solid mantle and liquid core to rotate differently.

Carter says these impacts would have produced enormous amounts of energy.

"If you release energy from impact it can set up large scale heating and fluid flows in the core, generating a magnetic field," says Carter.

"This theory also predicts a magnetic field about a hundred times weaker than the Earth's which is what we see today."